Postilla /Yale Peabody Museum of Natural History.

Peabody Museum

of Natural History

Yale University

New Haven, CT 06520

Postilla Number 175

2 May 1979

Diversification and Biogeography
of the One-Toed Horses
Onohippidium and Hippidion

Bruce J. MacFadden
Morris F. Skinner

Received 14 June 1977
Abstract

In this report we describe a large sample of
Onohippidium galushai, new species, from
the Late Hemphillian (Early Pliocene) of west-
ern Arizona, one ramus of Hippidion sp. from
the Early Hemphillian (Late Miocene) of the
Texas High Plains, and one ramus of cf. Hip-
pidion sp. from the Irvingtonian (Late Pliocene
to Early Pleistocene) of southern California.
These one-toed horses were previously
thought to have been confined to Plio-
Pleistocene deposits of South America
Onohippidium from Arizona POSSESSES a re-
tracted nasal notch, deep preorbital facial
fossae, and dental pattern diagnostic of South
American representatives of this genus. Hip-
pidion from Texas and California exhibit the
dental characters, especially in the deep ec-
toflexids, seen in South American representa-
tives of this genus. This is the first report of
these horses from North America. The pres-
ence of Onohippidium and Hippidion in
North America demonstrates that diversifica-
tion of these ‘“endemics” occurred prior to
dispersal to South America during the Pleis-
tocene great faunal interchange

© Copyright 1979 by the Peabody Museum of
Natural History, Yale University. All rights reserved
No part of this publication, except brief quotations
for scholarly purposes, may be reproduced without
the written permission of the Director, Peabody
Museum of Natural History.

Introduction

One of the most interesting groups of*fossil
horses is Known from Plio-Pleistocene de-
posits of South America. These horses, which
include Hippidion, Onohippidium, and
Parahipparion, are, so far as is known, charac-
terized by unusual morphological adapta-
tions. Both Hippidion and Onohippidium have
retracted nasal notches (Fig. 1). Along with
this retraction, the nasal bones remain elon-
gate and unreduced “splints.” Some other
mammals, such as tapirs, have retracted
nasal regions; which is an adaptation to the
presence of a proboscis. Retracted nasals

A

NASAL NOTCH
|

NASAL SPLINTS
\

B LACRIMAL FOSSA
i\

BUCCINATOR FOSSA
\

Fig. 1.

Skulls of Hippidion (A) and Onohippidium (B) show-
ing characters discussed in text (adapted from
Burmeister, 1875, and Sefve, 1912, respectively).
The nasal splints in the Onohippidium specimen are
broken anteriorly. Not to scale.

One-toed Horses

Postilla 175

A PREFOSSETTE POSTFOSSETTE

HYPOCONAL
GROOVE

PREPROTOCONAL
lh held "—HYPOCONE

POSTPROTOCONAL
VALLEY

PROTOCONE

B ISTHMUS,

METASTYLID

METACONID ,

ENTOCONID

HYPOCONULID

PARALOPHID————

é HYPOCONID
ren ere Ne ECTOFLEXID

Fig. 2.

Left upper (A) and left lower (B) cheek teeth of a
one-toed horse showing characters discussed in
text. Nomenclature follows Stirton (1941), Skinner
and Taylor (1967), and Skinner (MS).

are rare in fossil horses, although this feature
has been described for certain “Hipparion”
from Eurasia which apparently possessed a |
tapir-like proboscis during life (e.g., Sefve,
1927; Forstén, 1968; Sondaar, 1971). The only
other reported occurrence of nasal modifica-
tion in horses is in Onohippidium and Hippi-
dion. The type of nasal modification seen in
these two genera has not been observed in
other horses. It is possible that these horses
did not possess a proboscis because: 1) The
nasal bones are not posteriorly retracted as
seen in other mammals with a proboscis; 2)
The elongate nasal splints would have re-
stricted movement of soft anatomical struc-
tures. Simpson (1951, p. 147-148) states
that: “The nasal bones [in Hippidion]. . .must

have been well supported by a nasal septum
of cartilage and other tissues. .. .” It seems
that this type of nasal modification is related to
olfaction, but a more specific function is not
certain.

It is generally stated that Onohippidium

and Hippidion were derived from a group of
one-toed horses from North America (e.g.,
Matthew, 1924; Stirton, 1940). The purpose of
this report is to describe the first-known re-
cord of these two genera in Hemphillian and
Irvingtonian deposits of North America and to
discuss their biogeographic significance.

NEW MEXICO

|
Se ‘ 4 ee

Be colle

MEXICO

Fig. 3.

Fossil localities discussed in this report. 7. Unde-
scribed local fauna, unnamed rocks of the Ogallala
Group, Early Hemphillian (Late Miocene) of the
Texas Panhandle. 2. Wikieup local fauna (I.f.), Big
Sandy Formation, Late Hemphillian (Early Pliocene)
of western Arizona. 3. Sediments probably equiva-
lent in age to the Vallecito Creek |.f., Irvingtonian
(Late Pliocene-Early Pleistocene) of the Anza-
Borrego State Park of southern California.

Abbreviations

The following institutions are referred to in the
text:

F:AM Frick: American Mammals, De-
partment of Vertebrate
Paleontology, The American
Museum of Natural History

LACM Section of Vertebrate Paleontol-

ogy, Los Angeles County
Museum of Natural History

3 One-toed Horses

Postilla 175

All measurements of specimens are in mil-
limeters and are abbreviated as follows:

A=P Greatest anterior-posterior
length

il Greatest transverse width
(labial-lingual; external-
internal)

R Right side of tooth row

L Left side-of tooth row

@ Measurement approximate

The dental nomenclature used in this paper
and illustrated in Figure 2 follows Stirton
(1941), Skinner and Taylor (1967), and Skin-
ner (MS),

Systematic Paleontology

Class Mammalia Linnaeus, 1758
Order Perissodactyla Owen, 1848
Family Equidae Gray, 1821

Genus Onohippidium Moreno, 1891

Revised Distribution. Early Pliocene (Late
Hemphillian) of Arizona and Late Pliocene
and Pleistocene of South America (Sefve,
1912; Hoffstetter, 1952; Webb, 1976; this re-
Port).

Generic Diagnosis. Horse with nasal notch
that lies as far anterior as P?-P4 and as far
Posterior as the orbit. Nasal bones not re-
tracted and remain elongate splints. Well-

developed dorsal lacrimal fossa and no ven-

tral malar fossa. Incisors with cement-filled
Cups (infiditula). Protocone oval to rounded
with weak preprotoconal groove and well-
developed postprotoconal valley. Lower
Cheek teeth with either shallow ectoflexids
(externomedial valleys) and isthmuses in the
Permanent premolars or deep ectoflexids in
the deciduous premolars and molars. Very
Simple enamel plications. Monodacty!
Metapodials.

pnohippidium galushai, new species
ig. 4,

Fig. 4.

Onohippidium galushai, new species, from the Late
Hemphillian of Arizona. A. F:AM 100077, holotype,
left lateral view of partial skull. B. F:AM 100077,
holotype, left upper dentition. C. F:AM 100078, right
lower dentition, reversed.

Pliohippus sp., Lindsay and Tessman, 1974,
p.6.

Holotype. F:AM 100077, partial skull with
well-preserved dentition.

Hypodigm. Hundreds of specimens, which
consist of skulls, jaws, dentitions, and post-
cranials in the F:AM collection. There are
specimens possibly referable to this species
in other institutions, such as the small collec-
tions from the same locality at the University of
Arizona Laboratory of Paleontology, but these
were not examined during the present re-
search.

Locality and Age. Frick field crews collected
the F:AM specimens during the 1950s from
the Big Sandy Formation of Mohave County,
Arizona (Fig. 3). This formation, which con-
sists of lacustrine clay to marginal sand facies
deposited in an enclosed basin, outcrops
along the Big Sandy River from approximately
8.5 km north to 12 km south of the town of
Wikieup. Sheppard and Gude (1972) present
a detailed stratigraphic description of this
formation. Frick field crews collected a few
specimens from washes north of Wikieup, but
the bulk was collected from a locality approx-

4 One-toed Horses

Postilla 175

imately 12 km south of Wikieup, near the
southern outcrop margin. This locality is in-
cluded in Sheppard and Gude’s (1972, p. 6)
reference section R-3 located“... along a
wash that parallels Signal Road about 2,600 ft
northwest of the road, from NWY%NWY%SW%
sec. 29to NEY@SEVANW'% sec. 29, T. 15N.,R.
12 W., Greenwood Peak quad.” The fossil-
bearing zone is located in the upper part of
reference section R-3 (Sheppard and Gude,
1972: Galusha, MS). Galusha (MS) terms this
zone the ‘Clay Bank Quarry locality,” whichis
used in a broad sense to include fossils col-
lected locally within this zone. One large con-
centration within this locality is approximately
125 mwest of Clay Bank Quarry and is termed
“Bird Bone Quarry.” This fossil-bearing local-
ity and zone can be relocated with the use of
Frick field notes (Galusha, MS), Sheppard
and Gude (1972), and the U.S. Geological
Survey 7¥2'’ Greenwood Peak quad. During
the falls of 1976 and 1977 MacFadden and Dr
Noye M. Johnson (Dartmouth College) relo-
cated the Clay Bank Quarry locality and col-
lected samples for paleomagnetic and
fission-track analysis.

The mammalian assemblage from the Big
Sandy Formation is informally termed the
Wikieup local fauna (I.f.) in Tedford et al. (in
press). Apreliminary faunal list for specimens
collected by the University of Arizona field
crews is presented by Lindsay and Tessman
(1974). The Wikieup Lf. is largely undescribed
although some comparisons with as-
semblages of similar age have been made
(e.g., MacFadden, 1977; Tedford et al., in
press). Tedford et al. (in press) consider the
age of the Wikieup |.f. to be Late Hemphillian
(Early Pliocene), which corresponds to an ab-
solute time interval of approximately 4 to 5
million years ago.

Etymology. Named in honor of members of
the Frick Laboratory, Theodore and Marian
Galusha.

Specific Diagnosis. Same as for genus with
the following distinctions: Nasal notch re-
tracted to above P3-P4. Fossa between buc-
cinator and lacrimal fossae not as well de-
veloped as other Onohippidium (Fig. 4).

Description. The nasal notch is retracted
posteriorly to a position which lies over P°-P4
and is intermediate between the conditions
seen in North American Pliohippus and
Dinohippus and in South American Hippidion
and Onohippidium

There is a well-developed complex of fossae
on the cheek. The buccinator fossa Is an-
teriormost and in front of the cheek-tooth row.
Posterodorsal to the buccinator fossa there is
a faint preorbital depression; this is better de-
veloped in South American Onohippidium.
Posterior to this faint preorbital depression
there is a deep and well-defined lacrimal
fossa that is also seen in South American
Onohippidium. The infraorbital foramen lies
above P4—M".

The upper dentition of O. galushaiis very simi-
lar to South American Onohippidium. The in-
cisors have cement-filled cups. P' is reduced
in size. The premolars are larger in cross sec-
tion than the molars. The protocone is
rounded to oval with a weak preprotoconal
groove and a deep postprotoconal valley.
Occasionally, especially in the molars, the
protocone unites with the metaloph and
hypocone forming a cement-filled enclosure.
The hypocone is oval and usually slightly
smaller than the protocone and there is a dis-
tinct posterolateral hypoconal groove. The
pre- and postfossettes (lakes) are cresent-
shaped. The plications on the fossettes are
absent except for the posterior border of the
prefossette and anterior border of the post-
fossette.

The lower incisors have cement-filled cups.
The P1 and its alveolus are absent, which is
characteristic of advanced horses. The pre-
molars are usually larger than the molars. The
deciduous premolars show relatively deep
ectoflexids, between the protoconid and
hypoconid; however, these ectoflexids are not
deep enough to divide the isthmus that con-
nects the metaconid and metastylid to the
protoconid and hypoconid, respectively. The
permanent premolars have relatively shallow
ectoflexids that do not divide the isthmus. The

5) One-toed Horses

Postilla 175

enamel pattern is simple with few secondary
plications.

Discussion. The retracted nasal notch with
unreduced nasal splints, configuration of
preorbital fossae, and, to a lesser extent, sim-
ple dental pattern provide convincing evi-
dence for relating O. ga/ushai to the South
American representatives of this genus. So far
as is known, Onohippidium has not been iden-
tified elsewhere in North America. Other con-
temporaneous horses for which skulls are
known, i.e., Pliohippus and Dinohippus, have
characters different from those seen in
Onohippidium. Pliohippus has a different con-
figuration of preorbital fossae and no re-
tracted nasal notch, whereas Dinohippus has

a simple preorbital area and no retracted
nasal notch. There are numerous forms of the
Pliohippus-Dinohippus complex for which the
preorbital and nasal areas are not known.
Therefore, it would be most satisfying if the
Pliohippus-Dinohippus complex could be dif-
ferentiated from North American Onohip-
pidium solely on the basis of dentition. If this
dental differentiation were possible, then
Onohippidium might be identified at other
Late Hemphillian North American localities.
However, the dentitions of these contem-
poraneous forms are so similar, and there is
so much variation within “populations,” that
generic differentiation on this basis is virtually
impossible.

Fig. 5.

. 0 7 w
+ MILLIMETERS

Occlusal and lateral photographs of symphysis and left ramus of Hippidion sp., F:AM 101439, from the

Early Hemphillian of the Texas High Plains.

Genus Hippidion Owen, 1869

Revised Distribution. Early Hemphillian
(Late Miocene) of Texas, Irvingtonian (Late
Pliocene to Early Pleistocene) of California,
and Late Pliocene to Pleistocene of South
America (Sefve, 1912; Hoffstetter, 1952;
Webb, 1976: this report).

Generic Diagnosis. Horses with nasal notch
that lies above M'-M?. Nasal bones not re-
tracted and they remain elongate splints. Dor-
sal lacrimal and ventral malar fossae absent.
Incisors with cement-filled cups. Protocone
oval with weak preprotoconal groove and
well-developed postprotoconal valley. Lower
cheek teeth with deep ectoflexids in the pre-

6 One-toed Horses

Postilla 175

molars and molars. Enamel plications usually
more complex than in Onohippidium.
Monodacty! metapodials. For diagnostic
characters not represented in specimens dis-
cussed in the present report, see Burmeister
(1875), Sefve (1912), and Hoffstetter (1952).

Hippidion sp.
Figs. 5, 6, Table 1.

Referred Specimen. F:AM 101439, left ramus
and symphysis with R I,-1,, L };-Ms3

Locality and Age. The Frick field crew of 1941
collected this specimen from unnamed rocks
of the Ogallala Group, 10 miles north of Hig-
gins, Lipscomb County, Panhandle of Texas
(Tedford, personal communication, 1976; see
Fig. 3). The fauna from this locality is unde-
scribed, but itis similar in age to the Arnett |.f.
from adjacent Oklahoma described by Kitts
(1957). It is stratigraphically below the Hig-
gins I.f. described by Hesse (1940). Tedford
etal. (in press) consider the age of this locality
to be Early Hemphillian (Late Miocene).
Description. |, and|,have cement-filled cups
exposed at the occlusal surface (Fig. 5). 1, has
cups recessed below the occlusal surface.
There is no precanine diastema and C is
closely appressed to |3. Sexual dimorphism is
pronounced in fossil horse populations and
the small canine size in F:AM 101439 indi-
cates that this individual is a female. Posterior
to the symphysial dentition the ramus |s later-
ally constricted for the length of the post-
canine diastema, which measures 69.6 mm
from the posterior base of C to the anterior
base of P,. The mental foramen is approxi-
mately midway between C and P». The P; and
its alveolus are absent.

The molars are narrower transversely than the
premolars. The dental pattern of F:AM 101439
(Fig. 6) is similar to that of Hippidion illustrated
by Burmeister (1875). Pz is triangular-shaped
and similar in dental pattern to the rest of the
molars except for the addition of the
anteriormost conid, which is characteristic of
the P, in horses, whereas P,, through M; have
an anterointernal paralophid. The protoconid

and hypoconid are exteriorly convex. The ec-
toflexid is relatively deep in the cheek teeth of
F:AM 101489; this character is also seen in
other Hippidion such as those described by
Burmeister (1875), Sefve (1912), and Hoffstet-
ter (1952). When the ectoflexid is very deep it
subdivides the isthmus into the antroisthmus
and postisthmus (Skinner, MS). The
metaconids and metastylids are rounded and
subequal in size for a given tooth. There is
variation in the metaconid-metastylid com-
plex within the tooth row.

Fig. 6.
Occlusal view of Hippidion sp., F:AM 101439, show-
ing details of dental pattern (see Fig. 5).

InP,-P,and M2-M,the metaconid is joined to
the protolophid by a constricted antroisthmus
and the metastylid is joined to the hypoconid
by the postisthmus. In M, the isthmus is not as
constricted as in the other teeth and the
metaconid-metastylid complex is smaller and
more poorly defined than the other molars.
This variation in dental pattern of M, is proba-
bly a result of differential tooth wear. In the
premolars, the hypoconid and entoconid are
separated by a constriction. These same
parts are less distinct in the molars. In the Ms,
posterior to the hypoconulid, there is an iso-
lated conid that is also seen in other Hippi-
dion. This conid connects to the hypoconulid
during later stages of wear. F:AM 101439 is
similar in size to the smaller South American
Hippidion species. For dental and size differ-
ences, Figures 4and5, and Table 1 should be
compared to the illustrations and measure-
ments of Burmeister (1875), Sefve (1912), and
Hoffstetter (1952)

Discussion. The dental characters of F:AM
101439, especially in the diagnostic deep ec-
toflexids in both premolars and molars, pre-
sent aconvincing argument for its assignment

i One-toed Horses

Postilla 175

to Hippidion. The specific identification of the
Texas Hippidion is best deferred to a study
that incorporates other relevant specimens
The significance of this specimen with respect
to the biogeography of South American
“endemic” hoses will be discussed below.

Cf. Hippidion sp.
Fig. 7; Table 1.

Referred Specimen. LACM 3363, incomplete
left ramus with fragments of three cheek teeth,
probably P3—M, (Fig. 7)

Locality and Age. This specimen was col-
lected from LACM locality no. 1245-15 in sed-
iments probably equivalent to the Vallecito
Creek I.f. (Downs and White, 1965; Downs,
personal communication, 1977). This local
fauna occurs in the Matuyama reversed polar-
ity epoch below a normal event that repre-
sents either the Réunion’ or Olduvai. These
Constraints indicate an Irvingtonian (Late
Pliocene to Early Pleistocene) age for the cf.
Hippidion specimen that is bracketed by the
Gauss-Matuyama boundary at about 2.4 mil-
lion years ago and either the base of the Ré-
Union event at about 2.1 million years ago or
the base of the Olduvai event at about 1.85
million years ago (Lindsay, Johnson, and Op-
dyke, 1975; Opdyke et al., 1977).
Description and Discussion. LACM 3363 is
very poorly preserved. The teeth, probably
P-M,, are less hypsodont than contemporary
Equus. The dental pattern shows deep ecto-
flexids in both the premolars and molars, and,
therefore, we refer this specimen to Hippidion
The principal importance of LACM 3363 is in
the recognition of cf. Hippidion from Irving-
tonian sediments of North America.

Biogeography

The origin of the South American “endemic”
horses has been a matter of some interest in
the past. Many students of fossil mammals,

Including Matthew (1924) and Stirton (1940),
have recognized that this group was of North

pee eee ee

a

Fig. 7.

Occlusal view of cf. Hippidion sp., LACM 3363,
?P4-M,, from the Irvingtonian of southern California.
Enamel is blackened for contrast. Broken areas are
indicated by dashed lines (modified from an illustra-
tion prepared by Ms. F. Runyan of the LACM).

American origin. Horses are not found in
South America until the Uquian (Late Pliocene
to Early Pleistocene), roughly 2 million years
ago (Marshall et al., 1977), and their appear-
ance represents dispersal from North
America during the opening of the Central
American land bridge (Webb, 1976). Reed
(1950, p. 76-77), in his discussion of the Milk
Creek |.f. of Arizona, states that: “The horses
appear to be related to Pliohippus [Dinohip-
pus], and it has been suggested by Dr. R. A.
Stirton of the University of California that they
may be ancestral to the South American Hip-
pidium [sic] of the Pleistocene.” The great
“morphological distance” between these
North and South American horses has given
rise to speculation as to structural and
phylogenetic intermediates. Stirton (1940, p.
192) states that: “Upper Pliocene representa-
tives of this group, when found, should display
a moderately restricted narial notch in con-
trast to the extreme observed in Hippidium
[sic].” As predicted by Stirton, morphologi-
cally intermediate forms are now known from
North America as a result of the present re-
port

‘The existence, duration, and boundaries of the
Réunion event are presently uncertain (see, e.g.,
Opydke, 1972, and Watkins, 1972)

8 One-toed Horses Postilla 175

Table 1.

Tooth measurements (mm) for Hippidion.

Hippidion sp., F:AM 101439

Ri, Rl, Bale sa elk ele LC, lbedeaee) Leelee elie ee ee ESR lois ile Wh
A-P L@e qa) 0) 10.6 Hales ilies TAS@-s2@ Cie 2000. 2 es Oe cine. 20.)
v ins 14.6 TZ0 12.4 Wh! ilo: <4ee TAO) = OAR Os esos ee elaine
Cf. Hippidion sp., LACM 3363
oleae (Bales bE
A-P 30.2 28.4. 27.6@
ele abayiay Water SNS)

Abbreviation: @ = approximate.

As presently recognized, the one-toed horses
include the genera Pliohippus s. 7. (including
Astrohippus), Dinohippus, Onohippidium,
Hippidion (including Parahipparion), and
Equus s. 1. The phylogenetic relationships of
these genera have not been dealt with
adequately even in the classic studies of
Sefve (1912), Osborn (1918), Stirton (1940),
and Hoffstetter (1952). In fact, it might be
shown that the taxon “one-toed horses’ is not
a strictly natural, i.e., monophyletic group.
Based on the limited sample discussed inthis
report, we feel that a discussion of the interre-
lationships of Onohippidium, Hippidion, and
other one-toed horses should be deferred to a
more comprehensive study.

The major significance of the present study is
in the biogeography of Onohippidium and
Hippidion in light of the specimens described
here. The time of differentiation of Onohip-
pidium and Hippidion from their closest rela-
tives has not previously been discussed. lt
has been implied that diversification obvi-
ously occurred either in North America before
dispersal southward or in South America after
dispersal southward. We conclude that: 1)
Clearly differentiated forms of Onohippidium
and Hippidion are found in Hemphillian (Late
Miocene to Early Pliocene) deposits of North

America. 2) These genera subsequently dis-
persed to South America apparently during
the Pleistocene faunal interchange. 3) Cf.
Hippidion sp. is also found in Irvingtonian sed-
iments in North America

Acknowledgments

We thank Dr. Richard H. Tedford and Mrs. S.
Marie Skinner of the American Museum of
Natural History for their encouragement dur-
ing this research. Dr. Tedford brought the
specimen of cf. Hippidion sp. to our attention
and Dr. Theodore Downs of the Los Angeles
County Museum of Natural History kindly
loaned it for study. Messrs. Grant E. Meyer
and David Frailey offered criticisms that im-
proved earlier versions of the manuscript.
Messrs. Raymond J. Gooris and William K.
Sacco prepared Figures 4 and 5 respectively.
Mrs. Brigitte Meshako and Ms. Betsy Dabakis
typed portions of the manuscript. MacFadden
conducted some of this research during te-
nure of a faculty fellowship in the Department
of Geological Sciences at Columbia Univer-
sity. The University of Florida contributed
funds toward the publication of this research.

9 One-toed Horses Postilla 175

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The Authors

Bruce J. MacFadden. Formerly of: Depart-
ment of Geology and Geophysics, Yale
University, New Haven, Connecticut 06520.
Present address: Fiorida State Museum, Uni-
versity of Florida, Gainesville, Florida 32611.

Morris F. Skinner. Department of Vertebrate
Paleontology, The American Museum of
Natural History, New York, New York 10024.